It is common practice to clean particle-laden gases by passing them through a filter. When the gases have a low temperature, there are many porous materials that serve this purpose satisfactorily. Even when the gas has a higher temperature than the filter can handle, it is possible to cool the gas before filtering it. However, in some applications it is unsatisfactory to cool the gas, because of thermodynamic economy, or simply because it is inconvenient. For instance, in a large trash burning facility, it is desirable to remove particulate before releasing the combustion air to the atmosphere in order to significantly reduce the level of pollution. Cooling before filtering in such a case serves to add resistance to the flow of gas in the system and to add expensive complications.
Another situation in which removal of particles from hot gas would be desirable is in the case of the hot gas from a combustor flowing to a gas turbine; the particles in the gas tend to clog and to cause erosion on the turbine blades; further, the subsequent effluent from the turbine contributes to pollution. In this case, cooling the gas would upset the thermodynamic efficiency of the turbine, since the proper operation of a heat power machine requires as hot a gas as possible.
In order for a filter to be appropriate for the temperatures above 1,000.degree. C. (1,832.degree. F.) encountered in these two applications, it must have a number of fairly unusual properties. Besides being used at atmospheric pressure and high flow rates, the filter must be physically strong to be utilized in applications of high pressure with possible pressure differentials up to 400 pounds per square inch.
First of all, it must be physically strong, because the high volumes of gas which must pass through the filter will probably create sufficient force to impose significant mechanical strain on the filter.
Second, the filter must have a pore structure of effective diameter which can be set to a specific quantity and held to a very narrow range. Each application of the filter would require a specific effective diameter in order to optimize results and it is necessary that that specific diameter be essentially uniform and within very narrow tolerances across the entire filter. Otherwise, the filter will not efficiently carry out its filtering function with minimum resistance to flow. Finally (and most importantly), all of the qualities of this filter must be maintainable even if the filter is exposed to a very high temperature. For instance, in the normal range of 1,000.degree. C. (1,832.degree. F.) and at least as high as 1,500.degree. C. (1,832.degree.) and in either oxidizing, reducing or neutral atmospheric conditions.
Since most filter materials melt at temperatures substantially below this range, the only substance available is ceramic and attempts have been made to construct high temperature filters from such material. In the past, ceramic materials in the form of crushed aggregate have been used, but it has been difficult to obtain consistent or selected pore sizes. Bonding between particles, has been less than adequate, thus contributing to structural weakness. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide a filter having characteristics that permit operation with gases at temperatures over 1,000.degree. C., (1,832.degree.).
Another object of this invention is the provision of a filter having exceptional structural strength at high temperatures.
A further object of the present invention is the provision of a high temperature filter having consistent pore size throughout.
It is another object of the invention to provide a high temperature filter having a selected pore size within extremely narrow tolerance.
A still further object of the invention is the provision of a filter for use with hot gases, which filter is simple in construction, which can be easily manufactured from relatively inexpensive materials, and which is capable of a long life of service without replacement.
It is a further object of the invention to provide a filter having a structure that is capable of withstanding a severe treatment for the removal of accumulated particulate without damage.
With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.